PURPOSE: To enable the maintaining of analysis accuracy using an interference filter relatively easy to prepare, by employing an interference filter in which cavities are formed at two points with low refractive index material films or high refractive index material films arranged continuously.

CONSTITUTION: Interference filters 9a and 9b are formed by laminating a proper number of high refractive index material films H made of germanium and low refractive index material films L made of zinc sulfide on a substrate made of silicon. The filters are arranged to the Si LHLLHLHLHLLH as a whole. When measuring nitrogen oxide, a measuring gas is supplied to a measuring cell 1 while infrared rays from ray sources 3a and 3d are made incident into the measuring cell 1 and a comparison cell 2, being turned to intermittent rays with a chopper 8. A measuring component light is transmitted through the interference filters 9a and 9b to be incident into a rear light receiving section 5b from a front light receiving section 5a of a detector 4. An interference component light is reflected with the interference filters 9a and 9b and effect of a very small amount of interference component light transmitted therethrough is made up for by output signals of the front light receiving section 5a and the rear light receiving section 5b.